An optical isolator is a device which allows a light to pass through it in one direction only and blocks the light which attempts to propagate in the opposite direction. For example, by arranging an optical isolator on an exit end of a semiconductor laser, a light emitted from the semiconductor laser passes through the optical isolator and can be used as a light source for an optical fiber communication. Conversely, a light which attempts to be incident on the semiconductor laser through the optical isolator is blocked by the optical isolator and cannot be incident on the semiconductor laser. If the optical isolator is not placed on the exit end of the semiconductor laser, a reflected return light is incident on the semiconductor laser and degrades the oscillation characteristics of the semiconductor laser. In other words, the optical isolator has a function of both blocking the light which attempts to be incident on the semiconductor laser and maintaining a stable oscillation without degrading the characteristics of the semiconductor laser.
Not only in the above-described semiconductor laser, but also in optical active devices such as an optical amplifier, the incidence of an unintentional backward light degrades the operation characteristics of the device and may also cause some unintentional behavior. Since the optical isolator allows the light to pass through in one direction only, a backward light can be prevented from an unintentional incidence to the optical active device.
Conventionally, an interference-type optical isolator (a waveguide-type optical isolator), as shown in FIG. 1, has been proposed as an optical isolator suitable for integration with a semiconductor laser. This conventional optical isolator 101 comprises a waveguide layer 103 made of a semiconductor material on a compound semiconductor substrate 102, a waveguide 104 and tapered branching/coupling devices 105 on the waveguide layer 103. A clad layer 106 made of a magneto-optical material is formed on the waveguide layer 103, and a magnetic field applying means 107 for orienting the magnetization of the magneto-optical material to a predetermined direction is provided on the clad layer 106.
The above optical isolator (hereinafter referred to as a “waveguide-type optical isolator”) 101 is configured by employing a phase change (hereinafter referred to as the “nonreciprocal phase shift effect”) of a light wave with different magnitudes depending on a propagation direction occurring within two optical waveguides constituting an optical interferometer, so that light waves propagating through two optical waveguides have the same phase for forward propagating waves and are in phase opposition for backward propagating waves.
The operating principle of the waveguide-type optical isolator 101 is shown in FIGS. 2A to 2C. When the two light waves are in-phase, they are output from a central output end 111 in the tapered branching/coupling devices 105 provided on the output side of the waveguide-type optical isolator 101 due to the symmetry of the structure (FIG. 2B). On the other hand, when they are in phase opposition, the light waves that are input from the central output terminal (reflected return light) will form an anti-symmetric distribution at the tapered branching/coupling devices 105 provided on the input side of the waveguide-type optical isolator 101 (on the left side of the figure) due to the symmetry of the structure, and thus, the light waves will be outputted from an unnecessary light output terminals 112 provided on both sides of the central output terminal 110 instead of being output from the central output terminal (input terminal) 110 of the tapered branching/coupling devices 105 (shown in FIG. 2C). In other words, the light waves entered from the input end 110 of the left-side tapered branching/coupling device 105 are output from the output end 111 of the right-side tapered branching/coupling device 105, while the lights entered from the output end 111 of the tapered branching/coupling devices 105 do not return to the input end 110 of the left-side tapered branching/coupling device 105, thereby to allow the input end 110 to be isolated from the backward propagating waves.
The above-described operation is achieved by the structure shown in FIG. 2C. First, by making one of the interference optical paths longer than the other, a phase difference (a reciprocal phase difference) that is independent on a propagation direction is generated between two optical paths. A material (hereinafter referred to as a “magneto-optical material”) having a magneto-optical effect is arranged in the planar optical waveguide, and a magnetic field is applied externally to a direction (lateral direction) perpendicular to the propagation direction within the plane of the waveguide in order to orient the magnetization of the magneto-optical material, thereby to generate the nonreciprocal phase shift effect. On the basis of the relationship between the propagation direction of the light and the orientation of the magnetization, the nonreciprocal phase shift effect caused by the magneto-optical effect is determined. When the propagation direction is reversed with the magnetization direction maintained, the nonreciprocal phase shift effect is changed. Hereinafter, the difference in the nonreciprocal phase shift effect between the forward propagating waves and the backward propagating waves is referred to as the “nonreciprocal phase shift amount”.
In the waveguide-type optical isolator 101 shown in FIGS. 2A to 2C, since the magnetic field is applied to the two waveguides constituting the interferometer in an anti-parallel manner, the phase difference of the light waves when they have propagated through the two waveguides by the same distance coincides with the nonreciprocal phase shift amount. When a phase difference +φ is generated between the two waveguides for the forward propagating waves by the nonreciprocal phase shift effect, a phase difference −φ with the opposite sign is generated for the backward propagating waves.
In addition to the nonreciprocal phase shift effect caused by the magneto-optical effect, the two waveguides constituting the interferometer are designed so that a light propagating through the waveguide having the longer optical path has a larger phase change (hereinafter referred to as the “reciprocal phase difference”) exactly by “π/2” by providing an optical path length difference equivalent to a one quarter wavelength (λ/4) between the two waveguides. When the waveguide having the longer optical path is allowed to have a phase difference (nonreciprocal phase difference) “−π/2” caused by the nonreciprocal phase shift effect in comparison to the waveguide having the shorter optical path, the light waves propagating through the two waveguides become in-phase for the forward propagating waves (they are output from the central output end of the branching/coupling devices). When the propagation direction is reversed, since the sign of the nonreciprocal phase difference is reversed, the waveguide having the longer optical path is given a nonreciprocal phase difference “+π/2”. By adding the phase difference “+π/2” imparted due to the light path difference to the foregoing, the input to the tapered branching/coupling devices will be in the opposite phase state (i.e., phase difference π). As described above, a normal light and an abnormal light are thus isolated using the phase difference. One example of such a waveguide-type optical isolator is described in Japanese Patent No. 3407046 B2.
In addition, there exist the following Non-Patent Documents 1 to 4 on such a waveguide-type optical isolator.    Non-Patent Document 1: H. Yokoi, T. Mizumoto, N. Shinjo, N. Futakuchi and Y. Nakano, “Demonstration of an optical isolator, with a semiconductor guided layer that was obtained by use of a nonreciprocal phase shift”, Applied Optics, vol. 39, No. 33, pp. 6158-6164 (2000)    Non-Patent Document 2: Yokoi, Mizumoto, Shinjo, Futakuchi, and Nakano, “Operation demonstration of an optical isolator having a semiconductor waveguide layer”, IEICE Technical Report, OPE2000-10, pp. 3417-3421 (2000)    Non-Patent Document 3: T. Mizumoto, S. Mashimo, T. Ida, and Y. Naito, “In-plane magnetized rare earth iron garnet for a waveguide optical isolator employing non-reciprocal phase shift”, IEEE Trans. MAG, vol. 29, No. 6, pp. 3417-3421 (1993)    Non-Patent Document 4: T. Mizumoto and Y. Naito, “Nonreciprocal propagation characteristics of YIG thin film”, IEEE Trans. On Microwave Theory and Techniques, vol. MTT-30, No. 6, pp. 922-925 (1982)
Non-Patent Documents 1 and 2 report the actual trial manufacture of an optical isolator having a waveguide layer made of “GaInAsP” and an upper clad layer made of a magneto-optical material CeY2Fe5O12 (Ce:YIG). Measurement results regarding the optical isolator's characteristics are reported. These Non-Patent documents 1 and 2 report that an isolation ratio (=backward loss−forward loss) of 4.9 [dB] is achieved for the optical isolator at a wavelength 1.55 [μm]. In these Non-Patent documents, in order to clarify the reproducibility of the phase shift amount of a reciprocal phase shifter, an isolator structure having a reciprocal phase shifter with a reciprocal phase shift amount π, which is easy to measure, is manufactured (in the case of Non-Patent Documents 1 and 2, only insufficient operation as an isolator is achieved) in order to measure the reciprocal phase shift amount.
Further, Non-Patent Document 3 proposes a structure using tapered branching/coupling devices as optical branching/coupling devices constituting an interference system in an interference optical isolator using the nonreciprocal phase shift effect and discloses a waveguide design. The nonreciprocal phase shift effect is obtained by orienting the magnetization of a magnetic garnet as a magneto-optical material to the waveguide layer in the in-plane direction (the direction parallel to the substrate surface). In order to reduce the magnitude of the magnetic field required to orient the magnetization, the magnetic garnet is required to be grown that has in-plane magnetization characteristics. Non-Patent Document 3 discloses the growth conditions for growing a garnet (LuNdBi)3(FeAl)5O12 having the required in-plane magnetization characteristics using a liquid phase epitaxy method and shows the characteristics of the resultant magnetic garnet.
In Non-Patent Document 4, the amount of nonreciprocal phase shift generated for a TM-mode is measured through the magnetic garnet Y3Fe5O12 (YIG) in order to demonstrate the nonreciprocal phase shift effect.
In the above-described conventional waveguide-type optical isolators, however, as shown in FIGS. 3A and 3B, when the operating wavelength is a desired wavelength, the above-described phase difference occurs, and ideal operation as a waveguide-type optical isolator is shown, but when the operating wavelength changes, the magnitudes of the nonreciprocal phase shift amount and the reciprocal phase shift amount change for both the forward propagating waves and the backward propagating waves. In other words, errors resulting from the design conditions, wherein interference optical paths are in-phase for the forward propagating waves and are in phase opposition for the backward propagating waves, occur, thereby to degrade the characteristics of the isolator. In FIG. 3A, “θR” shows the reciprocal phase difference of an operating wavelength for the forward propagating waves and the backward propagating waves, “−θN” shows the nonreciprocal phase difference of the operating wavelength for the forward propagating waves, and “θN” shows the nonreciprocal phase difference of the operating wavelength for the backward propagating waves.
FIG. 3B shows a phase difference for the forward propagating waves and the backward propagating waves with the reciprocal phase difference and the nonreciprocal phase difference combined. When the inclinations of the changes in the phase differences with respect to the wavelength changes of the two light waves are compared to each other, the inclination of the backward propagating waves is larger than that of the forward propagating waves. This shows that when the difference between the operating wavelength and the designed wavelength increases, the degradation of the oscillation characteristics of a laser caused by insufficient suppression of the backward propagating waves, rather than by a diffusion loss of an incident light, significantly increases. First, for the diffusion amount of the backward propagating waves associated with changes in the operating wavelength, in a waveguide-type optical isolator having a designed wavelength 1.55 [μm], the characteristics of the changes in the operating wavelength and the diffusion amount (backward loss) of the backward propagating waves accompanying them is shown in FIG. 4.
In order to prevent the backward propagating waves from being irradiated on the optical laser, a certain level (generally, 30 [dB] is the required value) or more of the diffusion amount of the backward propagating waves is required. According to the characteristics shown in FIG. 4, the wavelengths showing a backward loss 30 [dB] or more are in the range of 1.54 to 1.56 [μm] (±0.01 [μm] with respect to the designed wavelength 1.55 [μm]). When the operating wavelength is not within that range, the backward propagating wave's diffusion effect on the waveguide-type optical isolator does not deliver a performance which satisfies the requirements.
On the other hand, for the insertion loss of the forward propagating waves associated with changes in the operating wavelength, in the waveguide-type optical isolator having the designed wavelength 1.55 [μm], the characteristics of the changes in the operating wavelength and the insertion loss (forward loss) of the forward propagating waves accompanying them are shown in FIG. 5.
The fact that when the operating wavelength deviates from the designed wavelength, the backward propagating waves have a larger inclination of a phase difference with respect to changes in the operating wavelength than the forward propagating waves (that is, the backward propagating waves have a higher wavelength-dependence than the forward propagating waves) has been described with reference to FIG. 4. A comparison of FIG. 4 with FIG. 5 highlights this fact. In the waveguide-type optical isolator having a designed wavelength 1.55 [μm], the insertion loss of the forward propagating waves when the operating wavelength is within the range of 1.44 [μm] to 1.64 [μm] is just 0.1 [dB], providing a remarkable difference from the operating wavelengths of 1.54 [μm] to 1.56 [μm], which still achieve the above-described backward propagating wave diffusion effect, thereby to satisfy the above-explained requirements.
As a result, since the wavelength-dependence of the isolation ratio defined by “backward loss−forward loss” is approximately determined by the wavelength-dependence of the backward loss, a conventional waveguide-type optical isolator cannot deliver the performance when the operating wavelength deviates from the wavelength range of 1.54 to 1.56 [μm].
In Non-Patent Documents 1 and 2, the obtained isolation characteristic is 4.9 [dB], which is insufficient for practical devices. In addition to this, the measurement wavelength of the isolation characteristic is 1.55 [μm], and the wavelength characteristics are not described. Furthermore, since a π/2-reciprocal phase shifter is used as the reciprocal phase shifter, the wavelength-dependence of the backward propagating waves is not eliminated.
In Non-Patent Document 3, as in the case of Non-Patent Documents 1 and 2, since a π/2-reciprocal phase shifter is used as the reciprocal phase shifter, the wavelength-dependence of the backward propagating waves is not eliminated. In addition to this, the magnetic garnet (LuNdBi)3(FeAl)5O12 in Non-Patent Document 3 has a Faraday rotation coefficient −600 [deg/cm] (with the wavelength 1.31 [μm]), which is approximately 13% of the Faraday rotation coefficient of the magnetic garnet Ce:YIG of the present invention, having −4,500 [deg/cm]. This will be described later. As a result, a problem arises in that the nonreciprocal phase shifter increases in length by nearly 7.5 times.
Although Non-Patent Document 4 is significant in that the nonreciprocal phase shift effect as a physical phenomenon is experimentally verified, it does not provide or describe an actual device such as an optical isolator.